The present invention relates to electrical switches for use in superconductive systems. More particularly, the present invention relates to switches for use in superconductive systems which store high levels of electrical and magnetic energy, particularly in the form of persistent current loops.
In conventional superconductive electrical systems in which persistent current loops are present, the cessation of current is typically accomplished through the act of heating a portion of the superconducting conductor to a point above its transition temperature. Once a portion of the current loop exhibits a finite resistance, electrical energy is dissipated in accordance with the well known I.sup.2 R law of power dissipation, where I is the current and R the resistance of the circuit. The heat that is generated, quickly causes adjacent areas of the conductor to also enter the resistive state and in a very short time the persistent current is quenched. However, when the level of current in the persistent loop is high, large levels of electrical energy must be dissipated in a short time in a small volume. When this is the case, a transition to the resistive state in order to turn off the current in the windings can result in damage to the superconductive material, particularly in the switch. Furthermore, in order for the switch portion of the superconductive wire to provide the necessary electrical resistance in its resistive state, the switch conductor must in some applications be quite long, 1,500 feet being a representative length. This length of wire must be compactly and firmly supported since small movements of superconductive conductors can cause them to switch into the normal, resistive state at an unacceptably low current level. Furthermore, because of the high levels of energy that must be dissipated, the material which is employed in the switch must have sufficient thermal mass to dissipate the energy in the current loop when the transition to the resistive state is made. Furthermore, the thermal conductivity of the switch must be high enough to prevent hot spots from forming that would otherwise damage the switch structure. However, since the switch is part of the persistent superconducting current loop, it must be maintained at a temperature below the transition point. With most materials that are presently available which exhibit superconductive properties, the transition temperature is typically below about 10.degree. K., although some transition temperatures are higher. Accordingly, the superconductive circuit must be contained within a coolant, such as liquid helium. However, the presence of a superconducting switch in such a coolant could produce the boiling of unacceptably large quantities of liquid helium. Typically boiled off helium vapor is vented to the atmosphere. Accordingly, it is highly desirable to provide thermal insulation between the superconductive conductor in the switch and the coolant in which the switch is disposed. Furthermore, since a switch is typically contained within a bath at a temperature of about 4.2.degree. K., it is necessary to insure that all materials employed in the switch are compatible with the coolant and the temperature ranges encountered in the environment. Accordingly, thermal expansion coefficients of the materials employed are important considerations. The switch sould also have a thermal insulating jacket that allows the switch body to be raised above the temperature of the coolant in which it is immersed. Even relatively small leaks of liquid helium through a thermally insulating jacket could allow unacceptably large heat losses. Insulating materials that could be employed, such as nylon and polytetrafluoroethylene (PTFE), which have desirably low thermal conductivities, also exhibit a high degree of shrinkage at liquid helium temperatures and they tend to leak. Accordingly, it is seen that a switch for use in superconducting current loops carrying high levels of electrical energy must be carefully designed to exhibit not only superconductivity, but also finite levels of resistance in a thermally harsh and varying environment and should be able to dissipate large quantities of electrical and thermal energy without developing hot spots when the switch is made resistive.